The present invention relates to a semiconductor radiation detector mounted on a nuclear medical diagnostic apparatus or the like.
A radiation detector for detecting a radiation (gamma ray) radiated from a radioisotope (RI) applied to an object to be examined is one of the most important constituent elements in the nuclear medical diagnostic apparatus. The performance of the radiation detector determines the apparatus performance such as the spatial resolution, energy resolution, counting efficiency, and the like.
At present, a scintillation detector is widely used as a radiation detector. The mainstream of the scintillation detector is an Anger detector using a combination of a scintillator (phosphor), a light guide, and a photomultiplier tube (PMT) array.
According to the detection mechanism of the scintillation detector, light generated by the scintillator upon incidence of a gamma ray is guided to the photomultiplier tube or photodiode by the light guide, and converted into an electrical signal by the photomultiplier tube or photodiode. The scintillation detector is very large in size and very heavy. Further, since the scintillation detector employs a two-stage structure of converting a gamma ray into light and light into an electrical signal, the energy resolution is low.
In recent years, a semiconductor detector has been introduced. In this semiconductor detector, a plurality of CdTe (cadmium telluride) semiconductor cells sandwiched between common and signal electrodes are arrayed in a matrix. When a gamma ray is incident on a semiconductor cell which receives a high voltage between the common and signal electrodes, electrons and holes generated in the semiconductor cell respectively move to the positive electrode (signal electrode) and the negative electrode. Induced charges are accumulated in a charge amplifier via the signal electrode. The charge amplifier outputs an electrical signal proportional to the energy of the incident gamma ray.
In this way, the semiconductor detector has a very simple detection mechanism of directly converting a gamma ray into an electrical signal by the semiconductor cell. Since the semiconductor cell can independently detect gamma rays, the energy resolution and counting efficiency are much higher than those of the scintillation detector. In addition, the semiconductor detector does not require any large-size, heavy scintillator, light guide, and photomultiplier tube, and thus can be downsized.
The conventional mainstream is a so-called lateral detector in which a gamma ray is incident on the semiconductor cell through the common electrode. Recently, a so-called vertical detector in which the semiconductor cell is arranged perpendicularly to the gamma ray incidence surface is proposed.
This vertical detector has various advantages compared to the lateral detector. First, the application voltage can be set lower in the vertical detector because the traveling distance of a gamma ray (photon) in the semiconductor cell is longer than in the lateral detector. Second, the pixel density, i.e., spatial resolution is higher in the vertical detector than in the lateral detector.
In the conventional vertical detector, however, since a plurality of small semiconductor cells must be arrayed in a matrix, the alignment precision decreases to generate a unique artifact. Note that in the lateral detector, since one semiconductor plate is divided into horizontal sections, the alignment precision is relatively high.
In the conventional vertical detector, since adjacent semiconductor cells must sandwich the signal electrode and insulating layer of one cell and the common electrode of the other cell, the blind zone, i.e., dead zone between the adjacent semiconductor cells increases to decrease the pixel density (decrease in sensitivity or spatial resolution).
The conventional vertical detector suffers the following problem. FIG. 1A is a plan view showing a conventional vertical semiconductor detector of one module. FIG. 1B is a side view when viewed from the arrow A in FIG. 1A. FIG. 1C is a sectional view taken along the line B--B in FIG. 1A. One module has a matrix of a plurality of semiconductor cells 101 each adhered to common and signal electrodes 102 and 104. A plurality of cell modules are arrayed to constitute a large-size semiconductor radiation detector.
In this case, the dead zone width between the cell of a given module and the cell of an adjacent module is much larger than the dead zone width between cells in one module, so a false image (artifact) is generated in the image.
A positron emission tomography (PET) uses a nuclide having a high energy of 511 keV. Photons having this high energy travel a long distance and may generate a photoelectric effect at a deep portion. As a result, a true incidence point and a point recognized on the apparatus side have a large error.
This problem will be described in detail. This problem occurs when the photon incidence angle is large, as shown in FIG. 2. When a gamma ray is incident on a certain semiconductor cell, and the cell outputs a signal, the apparatus recognizes the central point on the surface of this cell as an incidence point. When the photon incidence angle is large, the true incidence point and the recognition point (false incidence point) have a large error E.